The invention relates to thermostructural composite material parts, and more precisely to such parts in which there is at least a portion that is of small thickness, i.e. of thickness less than 2 millimeters (mm).
The invention applies more particularly to structural parts for aviation and space applications, in particular afterbody parts for gas turbine aeroengines, nozzle exhaust cones, . . . .
For such parts, proposals have been made to use thermostructural composite materials, i.e. composite materials having mechanical properties that make them suitable for constituting structural elements and that have the ability to conserve those properties at high temperatures. Such thermostructural materials are constituted in particular by carbon/carbon (C/C) composite materials (carbon fiber reinforcement and carbon matrix), and by ceramic matrix composite (CMC) materials, e.g. C/SiC materials (carbon fiber reinforcement with a silicon carbide matrix), C/C—SiC materials (carbon fiber reinforcement with a mixed carbon and silicon carbide matrix), or indeed SiC/SiC materials.
The fabrication of a C/C or a CMC material part usually comprises making a fiber preform having a shape that corresponds to the shape of the part that is to be obtained and that is to constitute the fiber reinforcement of the composite material, and then densifying the fiber preform with the matrix of the composite material.
In order to give the composite material the desired mechanical properties, while leaving sufficient access to its pores to enable it to be densified with a matrix, the fiber volume ratio, i.e. the percentage of the volume of the part or of the apparent volume of the fiber reinforcement that is occupied by the fibers, generally lies in the range 25% to 45%.
The fiber preform may be obtained by draping fiber plies, e.g. plies of fabric or sheets of unidirectional or multidirectional yarns, it being possible to superpose a plurality of plies and bond them together, e.g. by needling. The fiber preform may also be obtained from a fiber structure that is obtained by three-dimensional (3D) weaving or by multilayer weaving (a plurality of warp yarn layers linked together by weft yarns).
The fiber preform may be densified with a carbon or ceramic matrix by using a liquid technique or by chemical vapor infiltration (CVI). Densification by a liquid technique comprises impregnating the fiber preform with a liquid composition containing a resin that is a precursor of carbon or ceramic, and then polymerizing and pyrolyzing the resin in order to obtain a carbon or ceramic residue, with a plurality of consecutive impregnation, polymerization, and pyrolysis cycles possibly being performed. CVI densification is performed by placing the fiber preform in an enclosure and admitting a reaction gas into the enclosure under determined conditions in particular of pressure and temperature so that the gas diffuses into the preform and enables a deposit of the matrix material to be obtained as a result of one or more ingredients of the gas decomposing or as a result of a reaction between a plurality of its ingredients. For parts of special shapes, in particular of complex shapes, an initial step of consolidation by a liquid technique may be performed while using suitable tooling for holding the fiber preform in the desired shape, with densification then being continued without the aid of tooling, e.g. by CVI.
The above techniques are well known and have already been proposed for making parts for aviation and space applications that are exposed in operation to high temperatures, in particular turbine blades of aeroengines, afterbody portions of aeroengines such as secondary nozzles, mixers for bypass turbines, exhaust cones or nozzle flaps, and nozzles for rocket engines. Reference may be made in particular to the following documents in the name of the Applicant: WO 2010/007308, WO 2010/061139, WO 2010/061140, and WO 2008/104692.
When a part or a portion of a part is thin there is a problem of making a fiber preform that is suitable for obtaining the properties that are desired for the part that is to be fabricated.
The known technique that consists in forming a fiber preform by needling together superposed layers is hardly suitable since in order to obtain a fiber preform that is thin and that has uniform characteristics, it is necessary to begin by making a fiber structure of much greater thickness and then make use of its central portion only, thereby giving rise to large losses of material.
The known technique that consists in forming a fiber preform by a multilayer woven structure also presents drawbacks. Even when using carbon or ceramic yarns of the smallest commercially-available weight, the making of thin parts requires a reduction in the number of layers in a multilayer fabric, e.g. only two layers for a thickness of 0.75 mm, as shown in document WO 2008/104692, and that can affect the mechanical strength of the part. In addition, multilayer weaving produces a surface state that is irregular and produces an association of micropores (within the yarns) and macropores (between the yarns). For parts that are intended for aviation and space applications, such as for example stream mixers or nozzles, it is preferable to obtain a surface state that is smooth in order to avoid disturbing the fluid flow. The use of a preform with a surface state that is highly irregular gives rise, even after densification, to a part that presents significant surface relief. It is indeed possible to perform surface machining to improve that situation, however that leads to fibers being destroyed or laid bare, which is undesirable. In addition, the existence of macropores makes it inevitable that there will be residual irregular porosity after densification.
Document WO 94/12708 discloses a two-dimensional warp and weft fabric that is suitable for making reinforcing textures for composite material parts, the fabric being subjected to an operation in which the yarns making it up are spread apart by vibration in order to increase the fiber volume ratio. Spreading a two-dimensional fabric by vibration in order to eliminate holes in the fabric and thus increase the fiber volume ratio is also described in document EP 0 302 449.